Add date functions

src/data/courses/sql-mastery/chapters/scalar-functions/lessons/date-functions.md

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+---
+title: Date Functions
+description: Learn how to manipulate and format dates using SQL date functions
+order: 130
+type: lesson-challenge
+setup: |
+  ```sql
+  CREATE TABLE sale (
+      sale_id SERIAL PRIMARY KEY,
+      sale_date DATE NOT NULL,
+      amount NUMERIC NOT NULL
+  );
+
+  INSERT INTO sale (sale_date, amount) VALUES
+    ('2024-01-01', 100),
+    ('2024-02-02', 200),
+    ('2024-01-03', 300), 
+    ('2024-03-04', 400),
+    ('2024-01-05', 500),
+    ('2024-05-06', 600),
+    ('2024-03-07', 700),
+    ('2024-02-08', 800),
+    ('2024-01-09', 900),
+    ('2024-05-10', 1000);
+  ```
+---
+
+Date functions help you work with temporal data in SQL. These functions are essential for handling dates in reports, analyzing time-based data, and managing date-related business logic.
+
+We have already seen some date functions in the previous lessons, but let's look at some more examples in this lesson.
+
+## Extracting Date Parts
+
+Often you need to extract specific parts of a date (like year or month) for analysis or grouping:
+
+```sql
+-- Get year from date
+SELECT EXTRACT(YEAR FROM DATE '2024-03-15');    -- Returns: 2024
+
+-- Get month from date
+SELECT EXTRACT(MONTH FROM DATE '2024-03-15');   -- Returns: 3
+
+-- Get day from date
+SELECT EXTRACT(DAY FROM DATE '2024-03-15');     -- Returns: 15
+
+-- Get quarter
+SELECT EXTRACT(QUARTER FROM DATE '2024-03-15'); -- Returns: 1
+```
+
+## Date Formatting
+
+Converting dates to strings in specific formats is common for reporting and display purposes:
+
+```sql
+-- Format date as "Month DD, YYYY"
+SELECT TO_CHAR(DATE '2024-03-15', 'Month DD, YYYY');
+-- Returns: March 15, 2024
+
+-- Format as "MM/DD/YY"
+SELECT TO_CHAR(DATE '2024-03-15', 'MM/DD/YY');
+-- Returns: 03/15/24
+```
+
+> `DATE` keyword in the `DATE '2024-03-15'` is used to explicitly specify that the value being passed to the function is a date.
+
+## Adding and Subtracting Dates
+
+Date arithmetic is essential for calculating durations and future/past dates:
+
+```sql
+-- Add days to date
+SELECT DATE '2024-03-15' + INTEGER '5';
+-- Returns: 2024-03-20
+
+-- Subtract days from date
+SELECT DATE '2024-03-15' - INTEGER '5';
+-- Returns: 2024-03-10
+
+-- Add days to date
+SELECT DATE '2024-03-15' + INTERVAL '5 days';
+-- Returns: 2024-03-20
+
+-- Adding interval to date
+SELECT DATE '2024-03-15' + INTERVAL '1 year 2 months 3 days';
+-- Returns: 2025-05-17
+
+-- Add months to date
+SELECT DATE '2024-03-15' + INTERVAL '2 months';
+-- Returns: 2024-05-15
+
+-- Using INTERVAL for complex subtractions
+SELECT DATE '2024-03-15' - INTERVAL '2 weeks 3 days';
+-- Returns: 2024-02-27
+
+-- Combining multiple intervals
+SELECT DATE '2024-03-15' + INTERVAL '1 month' + INTERVAL '14 days';
+-- Returns: 2024-04-28
+
+-- Using INTERVAL with hours (converts to timestamp)
+SELECT DATE '2024-03-15' + INTERVAL '2 days 12 hours';
+-- Returns: 2024-03-17 12:00:00
+```
+
+## Date Differences
+
+Calculating the time between dates is a common requirement:
+
+```sql
+-- Get days between dates
+SELECT DATE '2024-03-15' - DATE '2024-03-10';
+-- Returns: 5
+
+-- Get difference between two dates
+SELECT AGE(DATE '2025-01-04', DATE '1992-07-25');
+-- Returns: 32 years 5 months 18 days
+-- Output is in INTERVAL format
+
+-- Extract year from INTERVAL
+SELECT EXTRACT(YEAR FROM AGE(DATE '2025-01-04', DATE '1992-07-25')) - 1;
+-- Returns: 31
+```
+
+> **Note:** `AGE` is a PostgreSQL specific function.
+
+## Current Date and Time
+
+SQL provides functions to get the current date and time:
+
+```sql
+-- Get current date
+SELECT CURRENT_DATE;
+
+-- Get current timestamp
+SELECT CURRENT_TIMESTAMP;
+
+-- Get current time
+SELECT CURRENT_TIME;
+```
+
+## Practical Examples
+
+Let's look at some real-world examples that combine multiple date functions. We'll use the following `sale` table:
+
+| sale_id | sale_date  | amount |
+| ------- | ---------- | ------ |
+| 1       | 2024-01-01 | 100    |
+| 2       | 2024-02-02 | 200    |
+| 3       | 2024-01-03 | 300    |
+| 4       | 2024-03-04 | 400    |
+| 5       | 2024-01-05 | 500    |
+| 6       | 2024-05-06 | 600    |
+| 7       | 2024-03-07 | 700    |
+| 8       | 2024-02-08 | 800    |
+| 9       | 2024-01-09 | 900    |
+| 10      | 2024-05-10 | 1000   |
+
+### Time Since Sale
+
+To calculate the time since a sale, we can use the `AGE` function.
+
+```sql
+-- Format sale dates and calculate time since sale
+SELECT
+    sale_id,
+    TO_CHAR(sale_date, 'Month DD, YYYY') as formatted_date,
+    AGE(CURRENT_DATE, sale_date) as time_since_sale
+FROM sale;
+```
+
+It will show the time since the sale in the format of `years months days`.
+
+### Sales by Month
+
+To group sales by month, we can use the `EXTRACT` function.
+
+```sql
+-- Group sales by month
+SELECT
+    EXTRACT(MONTH FROM sale_date) as month,
+    COUNT(*) as sale_count,
+    SUM(amount) as total_amount
+FROM sale
+GROUP BY EXTRACT(MONTH FROM sale_date)
+ORDER BY month;
+```
+
+### Sales from Current Month
+
+To find sales from the current month, we can use the `EXTRACT` function.
+
+```sql
+-- Find sales from current month
+SELECT *
+FROM sale
+WHERE EXTRACT(MONTH FROM sale_date) = EXTRACT(MONTH FROM CURRENT_DATE)
+AND EXTRACT(YEAR FROM sale_date) = EXTRACT(YEAR FROM CURRENT_DATE);
+```
+
+### Date Filtering
+
+When working with date ranges in reports or analysis:
+
+```sql
+-- Sales from last 30 days
+SELECT *
+FROM sale
+WHERE sale_date >= CURRENT_DATE - INTEGER '30';
+
+-- Sales between two dates
+SELECT *
+FROM sale
+WHERE sale_date BETWEEN DATE '2024-01-01' AND DATE '2024-12-31';
+```
+
+### Date Bucketing
+
+Grouping data by time periods is common in reporting:
+
+```sql
+-- Sales by quarter
+SELECT
+    EXTRACT(YEAR FROM sale_date) as year,
+    EXTRACT(QUARTER FROM sale_date) as quarter,
+    SUM(amount) as total_sales
+FROM sale
+GROUP BY
+    EXTRACT(YEAR FROM sale_date),
+    EXTRACT(QUARTER FROM sale_date)
+ORDER BY year, quarter;
+```
+
+### Age Calculations
+
+Calculating ages or durations:
+
+```sql
+-- Calculate customer age
+SELECT
+    customer_name,
+    date_of_birth,
+    extract(year from age(current_date, date_of_birth)) as age
+FROM customers;
+```
+
+> **Note**: While date functions are available in all major SQL databases, the syntax and function names can vary significantly. For example, Oracle uses `ADD_MONTHS` for adding months, while PostgreSQL uses the `INTERVAL` type. Some databases use `DATEADD` instead of the `+` operator. Always check your database's documentation for the exact syntax.
+
+In the next lesson, we'll explore conversion functions for transforming data between different types.